Method and system for monitoring environmental conditions

ABSTRACT

A sensing system, sensing method, and method of producing a sensing system capable of providing a cumulative measurement capability, such as in the form of a RFID tag capable of measuring cumulative heat and humidity for continuous monitoring of storage and shipping conditions of various goods. The system includes integrated circuitry and a plurality of sensing elements, preferably having cantilevered bimorph beams. Each sensing element is responsive to an environmental condition so as to deflect toward and away from open contacts in response to changes in the environmental condition. Each sensing element produces a digital output when it contacts and closes its open contacts. The integrated circuitry interfaces with the sensing elements so that the digital outputs of the sensing elements are processed to generate a system output of the sensing system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/680,718, filed May 13, 2005, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to methods and devices capableof monitoring environmental conditions, such as heat and/or humidity,including cumulative monitoring of heat and humidity. More particularly,this invention relates to a sensor in the form of a radio frequencyidentification (RFID) tag capable of measuring cumulative heat andhumidity for continuous monitoring of storage and shipping conditions ofitems in various applications, including supply-chain management ofperishable goods, pharmaceuticals, chemicals, and fresh agricultureproducts. The sensor RFID tag lends itself to automation andcost-effective supply chain management by using an RFID wireless linkfor transmission of cumulative sensor data to an RFID-reader(interrogator).

Environmental conditions, particularly accumulated exposure of an itemto heat and humidity over time, have a conspicuous effect on thelifetime and operational capabilities of a wide variety of goods andproducts, notable examples of which include perishable consumer andhealthcare goods such as food, medicine, vaccines, and blood bags, andmilitary ordnance such as explosives, propellants, and solid rocketfuel. In both commercial and military applications, assuringfunctionality and/or the absence of deterioration, spoilage, etc., isextremely crucial, since otherwise lives could be at risk andsubstantial economic losses could incur. This sensitive need requirescontrolled shipping and storage environments in conjunction withassigning conservative expiration dates. However, the functionality andstate of goods and products cannot be ensured without continuouslymonitoring environmental conditions, particularly heat and humidity, andthe cumulative effects thereof.

Existing temperature and humidity sensors can be primarily categorizedin two groups. A first of these is sensors based on change of color of alabel in response to humidity and temperature. These sensor labels arerelatively inexpensive and do not require a battery for power. However,significant shortcomings include being typically limited to indicatingor recording only maximum (or minimum) temperature or humidity levels.Furthermore, while sensor labels provide an easily observable visualoutput, they lack an electronic interface and thus are difficult todeploy in automated supply chain management networks.

The second group of existing temperature and humidity sensors can begenerally categorized as environmental parameter data logger modulesthat include individual humidity and temperature sensors, electronicchipsets for sensor interfacing and digitizing, a microcontroller,memory, a battery, and an external data communication link. Sensormodules of this type are generally capable of providing the necessaryinformation to assess the functional and qualitative state of goods andproducts through continuously monitoring heat, humidity, and theircumulative effects. However, sensor modules are generally costly, havelimited battery lifetime, are excessive in size for many applications,and require an additional (internal or external) software layer toprovide a cumulative output. The high cost of these modules,particularly at an item-level, is a major barrier to their wide use inmany applications.

In view of the above, it would be desirable if an environmental sensorsystem and method was available that overcame limitations andshortcomings of existing environmental sensor systems.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and system capable of providinga cumulative measurement capability that overcomes limitations ofexisting environmental sensing technologies and systems by using anarray of low-power digital micro-electro-mechanical-system (MEMS)sensing elements that can be cost-effectively batch manufactured andpackaged at wafer-level with integrated circuits. In a preferredembodiment, the invention provides a microsensor RFID tag capable ofcumulatively measuring one or more environmental conditions, such asheat and humidity, for continuous monitoring of storage and shippingconditions of items in various applications, including supply-chainmanagement of perishable goods, pharmaceuticals, chemicals, freshagriculture products, etc.

According to a first aspect of the invention, a MEMS-based digitalsensing system is provided that includes integrated circuitry and aplurality of sensing elements. Each sensing element comprises acantilevered bimorph beam and at least one set of open contactsconfigured for non-latching contact-mode operation with the beam. Thebimorph beam of each sensing element is responsive to an environmentalcondition so as to deflect toward and away from the open contactsthereof in response to changes in the environmental condition.Furthermore, the bimorph beams are configured to contact and close theirrespective open contacts at different levels of the environmentalcondition. Each sensing element produces a digital output when itsbimorph beam contacts and closes its open contacts, for example, as aresult of being sensed resistively, capacitively, etc. The integratedcircuitry interfaces with the sensing elements so that the digitaloutputs of the sensing elements are processed to generate a systemoutput of the sensing system.

In a preferred embodiment, the system output is a time-weighted outputthat cumulatively reflects the sensing elements whose bimorph beams havecontacted their contacts over time. More preferably, the system outputis cumulative and represents the time-weighted sum of the digitaloutputs from the highest responding sensing elements, in other words,the sum of the multiplication products of the digital outputs of thehighest responding sensing elements and the contact durations of thosesensing elements. Furthermore, the integrated circuitry preferablyinterfaces with the sensing elements such that the digital outputs ofthe sensing elements are not equal, but instead a sensing element thatproduces its digital output at a higher level of the environmentalcondition has a greater digital output than a second sensing elementthat produces its digital output at a lower level of the environmentalcondition. This aspect of the invention can be achieved with the use ofa variable digital clock assigned to the highest responding sensingelement at any given time and a counter whose output represents thecumulative measurement for the environmental condition. The preferredrelationship between clock rate and the sensing element is such that theclock rate increases with increasing levels of the environmentalcondition. Accordingly, the cumulative system output is nonlinearrelative to the environmental condition being sensed, and thereforeemulates the relative damage that can be caused, for example, to aproduct by increasing levels of the environmental condition.

Another preferred aspect of the invention is that the digital outputs ofonly a subset of the sensing elements are processed by the integratedcircuitry to generate the system output of the sensing system. By usingonly a subset of the sensing elements, the system output can be based onthose sensing elements identified during fabrication of the sensingsystem to have desirable operation characteristics, e.g., for theparticular range of environmental condition levels anticipated for thesensing system.

According to a second aspect of the invention, a digital sensing methodis provided that employs integrated circuitry and a plurality ofcontact-mode sensing elements responsive to an environmental condition,The sensing elements are operable to close sets of open contacts atdifferent levels of the environmental condition to individually producedigital outputs. The integrated circuitry is then used to interface withthe sensing elements so that the digital outputs of the sensing elementsare processed to generate a system output of the sensing system.Preferred aspects of the sensing method include the preferredoperational capabilities described above for the sensing system of thisinvention.

According to a third aspect of the invention, a method is provided forproducing a MEMS digital sensing system that operates with a only subsetof a plurality of contact-mode sensing elements. The method includesfabricating integrated circuitry and the plurality of sensing elements,with the sensing elements being responsive to an environmental conditionand operable to close at least one pair of open contacts at differentlevels of the environmental condition to individually produce digitaloutputs. Responses of the sensing elements to different levels of theenvironmental condition are determined, after which a subset is selectedof the sensing elements that produce digital outputs corresponding to apredetermined range of levels of the environmental condition. Theintegrated circuitry is then used to process the digital outputs of onlythe subset of the sensing elements to generate a system output of thesensing system while a remainder of the sensing elements is ignored bythe integrated circuitry when generating the system output. The subsetof sensing elements are preferably selected on the basis of havingdesirable operation characteristics, e.g., for the particular range ofenvironmental condition levels anticipated for the sensing system. Inthis manner, calibration of the sensing system is unnecessary, contraryto conventional practices associated with electronic sensors of theprior art.

In view of the above, it can be seen that the MEMS digital sensingsystem, sensing method, and fabrication method of this invention arewell suited for implementing automated and cost-effective supply chainmanagement by processing the digital outputs of the sensing elements togenerate one or more cumulative system outputs, such as cumulativetemperature and humidity outputs, and then using an RFID wireless linkfor transmission of these cumulative outputs to a RFID reader(interrogator). The advantages of the invention can be further realizedby fabricating the sensing elements using CMOS-compatible batchfabrication process technologies. By fabricating large arrays of MEMSsensing elements and selecting only certain elements to generate thesystem output based on the desirable operational characteristics of theselected elements, the present invention enables post-manufacturingsensor testing to be greatly simplified by avoiding the need forpost-manufacturing calibration or otherwise adjusting sensingcharacteristics of individual sensors that tend to deviate from targetedcharacteristics due to manufacturing process variations and tolerances.Finally, another advantage of the invention is the ability to employ anelectronic system architecture that in conjunction with the sensingelements results in ultra-low power operation and extended battery life.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a perspective view of a microsensor RFIDtag containing a sensor array package in accordance with a preferredembodiment of this invention.

FIG. 2 represents a cross-sectional view of the sensor array package ofFIG. 1.

FIG. 3 is a perspective view of an individual temperature sensingelement of the sensor array package of FIG. 2.

FIGS. 4 and 5 represent the response of a sensing beam of thetemperature sensing element of FIG. 3 to an increase and decrease,respectively, in temperature.

FIG. 6 is a graph plotting the effect that the geometrical design of thesensing beam has on temperature sensitivity of the sensing element.

FIG. 7 is a perspective view of an individual humidity sensing elementof the sensor array package of FIG. 2.

FIG. 8 represents the response of a sensing beam of the humidity sensingelement of FIG. 7 to a change in humidity.

FIG. 9 schematically represents two arrays of sensing beams withdifferent sets of beams being selected for utilization in differentsensor array packages in accordance with a fabrication yield-enhancementtechnique of this invention.

FIGS. 10A through 10E schematically represent the functionality of anoptional power-efficient temperature compensation scheme for thehumidity sensing element of FIG. 7.

FIG. 11 is a simplified block diagram of the interface electronics ofthe microsensor RFID tag of FIG. 1.

FIG. 12 is a pair of graphs that show the output of a counter withcontrolled clock speed used to indicate the cumulative temperature orhumidity measurements to provide an efficient IC implementation of themicrosensor RFID tag of this invention.

FIG. 13 schematically represents a low-power sensor readout front-endcircuit for use with the temperature and humidity sensing elements ofthis invention.

FIGS. 14 through 20 represent steps of a post-CMOS integrated sensorfabrication process by which arrays of sensing elements can befabricated on the same substrate as the interface electronics.

FIGS. 21 through 24 represent steps of a wafer-level packaging processby which arrays of sensing elements and the interface electronics can bepackaged.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically represents a microsensor RFID tag 10 in accordancewith a preferred embodiment of the invention. FIG. 1 representscomponents of the tag 10 as including a substrate 12 that carries one ormore sensor arrays 14, interface electronics 16, a clock generator(timer crystal) 18, an RFID front-end transceiver 20, an antenna 22, anda battery 24. While the tag 10 of this invention will be discussed withparticular reference to humidity and temperature sensing, those skilledin the art will appreciate that technological aspects of the tag 10 canbe implemented with other types of sensors, including chemical,shock/vibration, tilt, pressure, acceleration, and biological sensors.

The substrate 12 can be of any suitable construction and material, suchas those currently used in RFID and/or electronics technologies, andtherefore will not be discussed in any detail here. Other than as notedbelow, the clock generator 18, transceiver 20, antenna 22, and battery24 can also be of known construction and design, and therefore will onlybe discussed to the extent necessary for those skilled in the art tounderstand and implement the invention. In contrast, the sensor arrays14 and interface electronics 16 are preferably configured to providecertain advantages particular to the present invention. FIG. 2represents a cross-sectional view of a package 50 containing the sensorarrays 14. Though not shown, the interface electronics 16 are alsopreferably enclosed within this package 50, as will be described withreference to FIGS. 14 to 20. Two sensing elements 26 and 28 are shown inFIG. 2, representative of two arrays 14 of sensing elements 26 and 28within the package 50. Each element 26 and 28 is represented as having acantilevered beam 30 and 42, respectively, which are bimorphic accordingto a preferred aspect of the invention. While the invention will bediscussed with specific reference to the package 50 containing twoarrays 14 of sensing elements 26 and 28, and the sensing elements 26 and28 being specifically adapted to sense temperature and relativehumidity, respectively, those skilled in the art will appreciate thatthe package 50 could contain any number of arrays of sensing elements,and the sensing elements 26 and 28 can be configured to sense otherenvironmental conditions to which the package 50 might be subjected,including chemicals, shock/vibration, tilt, pressure, acceleration,biological agents, etc. Such capabilities can be achieved by usingappropriate materials to form the bimorphic beams 30 and 42 of thesensing elements 26 and 28, as will be understood from the followingdiscussion.

FIGS. 3 and 7 represent perspective views of individual temperature andhumidity sensing elements 26 and 28, respectively. According to apreferred aspect of the invention, each sensing element 26 and 28 is abimorph MEMS structure that functions as a switch in response to eitherthe temperature or humidity of the environment surrounding the tag 10.As such, the sensing elements 26 and 28 are able to extract the energyneeded for mechanical switching from the environment itself, therebydrastically reducing the power required to sense temperature andhumidity. The mechanical switching operation of an array of the sensingelements 26 and 28 is inherently digital and can be converted to anelectrical signal using simple compact front-end circuitry 60 (FIG. 13).In a preferred embodiment, the front-end circuitry 60 is able to makeuse of a minimal number of transistors and may dissipate less than onethousand picowatts per sensing element 26 and 28, resulting in a totalelectrical power dissipation on the order of nanowatts. As such, thesensing elements 26 and 28 of this invention are capable of operating ina manner that avoids the limitations of many existing IC-based sensors,which even if designed for lower power consumption are incompatible forcontinuous monitoring of temperature and humidity over a period of fewyears if relying on the energy capacity of existing miniature batteries.Furthermore, as will become evident from the following discussion, apreferred system architecture of the tag 10 is optimized for cumulativemeasurements using a simple and elegant counter-based architecture thatdoes not require any power consuming arithmetic logic units (ALU's). Incombination, these features significantly decrease the complexity of thetag 10 and its electronics to attain reductions in size, cost, and powernot attainable with current commercial embodiments of environmentalsensors.

With reference now to FIG. 3, the bimorph cantilevered beam 30 of anindividual temperature sensing element 26 is represented as beingfabricated to include two thin films 32 and 34 having differentcoefficients of thermal expansion (CTE). Metals such as aluminum andgold are believed to be preferred for the films 32 and 34, respectively,though it is foreseeable that other material combinations could be used,including metals and nonmetals. While the films 32 are shown as beingpositioned one atop the other to yield a vertical bimorph stack(vertical being normal to the surface of the substrate 36), it should beunderstood that the films 32 and 34 could be arranged side-by-side toyield a horizontal (lateral) bimorph stack (again, relative to thesurface of the substrate 36). Furthermore, as understood by thoseskilled in the art, the beam 30 could include additional layers/films,such as adhesion layers to promote adhesion of the films 32 and 34 toeach other, and stress compensation layers to improve the distributionof any processing-induced strain within the beam 30. As examples, if thefilms 32 and 34 are aluminum and gold, suitable adhesion layer materialsinclude titanium and chromium, which in some cases may also be suitablefor use as a stress compensation layer. It is also within the scope ofthe invention to pattern some of the layers that form the beam 30 forthe purpose of modifying their properties, including response totemperature and/or other environmental conditions, electricalconductivity for use as electrical contacts, etc. As such, it should beunderstood that the beam 30 comprises layers of various materials that,in combination, yield a bimorphic effect One end of the beam 30 isanchored to a suitable substrate 36, preferably a conventional CMOScircuit substrate in which the interface electronics 16 is alsofabricated, as discussed later. The opposite end of the beam 30 is shownas being suspended between two sets of open contact pairs 38 and 40. Thebeam 30 may have electrically-conductive layers (not shown) for makingelectrical contact with the contact pairs 38 and 40. It can be readilyappreciated that the structure of the sensing element 26 is simple andcompatible with post-CMOS processing, and that very large, high-densityarrays of such sensing elements 26 can be fabricated in a very smallarea.

As a result of its multilayer, bimorphic construction, the cantileveredbeam 30 freely deflects with temperature change due to the CTE mismatchof the films 32 and 34. FIGS. 4 and 5 illustrate an example of thesensing element 26 of FIG. 3 in which the beam 30 has a vertical bimorphstack, with its upper film 32 having a higher CTE than the lower film34, for example, an aluminum film 32 over a gold film 34. A non-latchingcontact-mode switching function is provided when the portion of the beam30 between the contact pairs 38 and 40 touches one of the pairs 38 or40, completing an electrical circuit containing that particular pair 38or 40. The temperature sensitivity of the cantilevered beam 30 of anygiven temperature sensing element 26 can be analytically obtained basedon structure geometries and material properties. The tip deflection in abimorph beam with no intrinsic stress is calculated by:Y _(tip)=3ΔT(α₂−α₁)(t ₂ +t ₁)L ²/(t ₂ ²(4+6t ₁ /t ₂+4(t ₁ /t ₂)²+(E_(1/) E ₂)(t _(1/) t ₂)³ +E ₂ t ₂ /E ₁ t ₁))where ΔT is temperature change, t₁ and t₂ are the thicknesses of thefilms 32 and 34, α₁ and α₂ are the CTE's of the films 32 and 34, and Eis the Young's modulus of elasticity of the films 32 and 34. Becausesensitivity is independent of the beam width, the widths of the beams 30can be minimized to reduce the size of the array to the extent thatmanufacturing reliability allows. FIG. 6 shows a plot of beam lengthsneeded to close a gap of “d” over a range of temperatures for sensingelements 26 whose beams 30 are formed by aluminum and gold films 32 and34 (respectively) having equal thicknesses (t₁ and t₂) of either 0.5 or1 micrometer. FIG. 6 evidences the feasibility of fabricatingtemperature sensing elements 26 whose beams 30 will make contact withtheir respective contact pairs 38 and 40 within a temperature range thatencompasses temperatures typically encountered during shipping andstorage. From FIG. 6, it can also be seen that a switching function at adesired temperature setpoint (threshold) can be obtained by fabricatingan array of sensing elements 26 whose beams 30 are intentionally ofdifferent lengths, with longer beams 30 being more sensitive totemperature and resulting in contact with one of the sets of contactpairs 38 and 40 at progressively smaller temperature changes withincreasing beam lengths. Scaling of the feature sizes of the beams 30improves the achievable measurement resolution in addition to the diesize reduction. The process patterning feature size for a given desiredtemperature resolution is approximately equal to the minimum slope ofthe beam length vs. temperature plot of FIG. 6 multiplied by the desiredtemperature resolution. A patterning resolution of less than onemicrometer is believed to be adequate to achieve desirable operation ofthe tag 10, such as ten-bit resolution.

Similar to the temperature sensing elements 26, the humidity sensingelements 28 of this invention also operate on the basis of a bimorpheffect, which causes deflection in its cantilevered beam 42 that can bedetected by switching contacts. As represented in FIG. 7, thecantilevered beam 42 comprises a pair of thin films 44 and 46 thatexhibit different humidity-induced expansion characteristics. Forprocessing purposes, the lower film 46 is preferably formed of the samematerial as the upper film 32 of the temperature sensing element 26,e.g., a thin metal film that does not exhibit any appreciablehumidity-induced expansion, such as aluminum or gold. In contrast, theupper film 44 of the beam 42 is formed of a material that exhibits anotable response to humidity. Certain polymer materials are well suitedfor the upper film 44, a particular example of which is the PI-2730series of low-stress G-line photodefinable polyamides available from HDMicrosystems. For feature sizes below about three micrometers, a morepreferred material for the film 44 is believed to be a higher densitylow-stress I-line polyimide such as HD-8000, also available from HDMicrosystems. In most cases, one open contact pair 48 is adequate forthe humidity sensing elements 28, since the beams 42 only deflect in onedirection (downward in FIGS. 7 and 8) due to humidity-induced expansionof the top beam film 44 (assuming the humidity sensing element 28 isfabricated in a dry (e.g., oven) environment). The contact pair 48 couldbe placed above the beam 42, similar to the upper contact pair 38 ofFIGS. 3 through 5, if the lower film 46 within the beam 42 is made froma material more responsive to humidity than the upper film 44. Inaddition, it should be understood that the response of the beam 42 tohumidity can be altered by completing its fabrication in an environmentcontaining a controlled level of humidity, in which case two opencontacts for each sensing element might be needed. Finally, as with thebeam 30 of the temperature sensing element 26, the beam 42 of thehumidity sensing element 28 can be formed of multiple layers of avariety of different materials, both metallic and metallic, includingadhesion-promoting, stress-distributing layers, and electrical contactlayers, as well as patterned layers for the purpose of modifying theresponse of the beam 42 to humidity and other environmental conditions.

In the past, MEMS humidity sensors have employed various transductiontechniques including capacitive based on detecting a change indielectric constant of a humidity sorbent polymer, resonant based ondetecting a change in mass after humidity absorption, and bimorphicbased on detecting a change in resonant frequency of a diaphragm ordetecting bimorph strain using piezoresistors. In general, bimorphichumidity sensors operate on the basis of a sorption-induced volumeexpansion of a hygroscopic layer within a bimorph structure. Theabove-noted polyimides have coefficients of volume expansion on theorder of 10⁻⁵/%RH, which is quite adequate for the sensing of humidityin accordance with this invention. It should be understood thatidentification of the most suitable polymer materials, film design, andoptimum sensitivity are desirable, as is the ability to improve andadjust the humidity sorption of the upper film 44, such as by ionbombardment of the upper film 44 if formed of a preferred polyimide.

While bimorphic temperature and humidity sensors are known, and thebimorphic effect has been employed to make latchable MEMS temperatureswitches, MEMS capacitive and piezoresistive sensors, and MEMS humiditysensors in the past, the present invention applies this known technologyin the form of simple contact-mode (non-latching) switches capable ofbeing fabricated in high-density, compact arrays, preferably with thecapability of being monolithically integrated with CMOS circuitry, in amanner that directly impacts system level performance parameters ofpower, size, and cost. For example, the substrate 36 is preferablyfabricated to contain sufficiently large arrays of each sensing element26 and 28 so that the tag 10 only requires the operational use of afraction of each array of elements 26 and 28. As discussed in moredetail below, the elements 26 and 28 chosen for use are selected toensure proper performance of the tag 10 over a range of environmentalconditions anticipated for the tag 10.

In practice, the thin films 32, 34, 44, and 46 of the beams 30 and 42have intrinsic stresses, which cause the beams 30 and 42 to deflect atroom temperature and humidity conditions, thus shifting the switchingthresholds of the individual sensing elements 26 and 28. Such an effectis undesirable, particularly since controlling thin film stresses duringmanufacturing can be difficult. This problem can be reduced in part byminimizing stresses in the initially deposited films 32, 34, 44, and 46through the proper choice of materials. Aluminum and gold are very goodcandidates for the films 32, 34, and 46 for this reason. If deposited bysputtering, the intrinsic stresses of metal layers (e.g., films 32, 34,and 46) can be further minimized by optimizing the deposition pressureand bias conditions. A post process temperature annealing step can alsobe performed to relieve intrinsic stresses to some extent, and helps topreclude sensor drift due to long term stress relaxation.

Because arrays containing in excess of a hundred to thousands of sensingelements 26 and 28 can be readily fabricated using MEMS technology, thesensing approach of this invention enables the tag 10 to have a largeredundancy of sensing elements 26 and 28 that enhances yield without anynoticeable cost penalty. Therefore, according to a preferred aspect ofthe invention, significant yield enhancements can be achievedfabricating the sensing elements 26 and 28 in sufficiently large arrays,and then selecting only a subset of elements 26 and 28 from each arrayfor actual use by the tag 10 to perform the temperature and humiditysensing functions. Such an approach is represented in FIG. 9, in whicheight of twenty sensing elements (26 or 28) are represented as beingselected on the basis of having suitable operating characteristicswithin a targeted range of the particular environmental condition ofinterest, e.g., temperature or humidity. While the selection of eight oftwenty elements 26/28 is represented in FIG. 9, it should be understoodthat the number selected and the size of the array are not limited bythis example, and that performance improves as the numbers of sensingelements 26 and 28 are increased in each array. The lefthand side ofFIG. 9 represents a situation in which controlled processing hasresulted in intrinsic stresses such that the as-fabricated elements26/28 of the array have acceptable operating characteristics. Theelements S1-S8 would be suitable as the operational switches for thesensor arrays 14 if the stresses remain within expected levels, andredundancy is obtained because a larger number of elements 26/28 arepresent that cover a wider range and provide higher resolution of theenvironmental condition than what is required by the sensor application.

After further manufacturing and performing an optional annealing, thedeflection of some of the elements 26/28 may be more or less thanexpected as a result of manufacturing variations, including geometry,surface characteristics, process-induced stress variations, etc.,resulting in switching at lower or higher thresholds (e.g., roomtemperature or low humidity). The states of these elements 26/28 can bedetected by a single readout during initial testing. The stressvariations and post-fabrication thresholds of the elements 26/28 withintheir respective arrays can then be determined based on the final stateof the elements 26/28, followed by a digital reassignment (selection) ofthe elements 26/28 that will be operational within the tag 10. In thismanner, a certain subset of each array of elements 26 and 28 can beselected to correct for any shifts in operational characteristics,including those attributable to stress and manufacturing variations. Ina preferred embodiment, in which by geometrical design the elements26/28 are arranged in their arrays so that their thresholds differlinearly along the length of the array, the general effect of stress andmanufacturing variations will appear as an offset. Therefore the arrayreassignment is a shift operation, in which a single block of adjacent(side-by-side) elements 26/28 is selected, minimizing the complexity ofthe control logic circuitry necessary to perform this operation andrelaxing the on-chip non-volatile memory requirements of the tag 10.

The approach to sensor fabrication described above significantly differsfrom conventional sensor manufacturing approaches in which tolerances ofthe manufacturing processes are tightened and post fabricationcalibration is employed to perfect the characteristics of individualsensors, for example, by physically modifying and/or electronicallycompensating a sensor to alter its response or output relative to theparameter being sensed. It is important to note that the sensorfabrication and sensing scheme of this invention is made technically andeconomically practical by the use of relatively low-cost butfully-integrated MEMS sensing elements 30 and 42, by which costeffectiveness is promoted by the presence of redundant sense elements 30and 42 that exhibit a range of sensitivities to the environmentalcondition(s) of interest.

As known in the art, a significant issue concerning humidity sensors istheir temperature sensitivity. In the past, this sensitivity has beenelectronically compensated using analog or DSP approaches, each ofwhich, in terms of power requirements, is very demanding. Depending onthe materials used, the humidity sensing elements 28 of this inventionmay also exhibit a temperature sensitivity due to the CTE mismatch ofthe films 44 and 46. The overall effect of CTE mismatch has beeninvestigated through FEM simulations, which have shown that highertemperatures correspond to reduced humidity sensitivity, i.e., switchingat higher humidity levels than intended. However, it is believed thatsuitable materials for the films 44 and 46 of the beam 42, such as thepreferred gold, polyamide, and polyimide materials, can be chosen toavoid the need for temperature compensation in many applications. As anoptional aspect of this invention, FEM simulations have been employed toshow that by properly designing the length of a humidity sensing beam42, the deflection at a given relative humidity percent (RH%) can bemaintained the same regardless of temperature variations using apower-efficient temperature compensation scheme shown in FIGS. 10Athrough 10E.

The represented temperature compensation scheme is based on the use ofmultiple contact-mode humidity switching elements 58 in parallel to forman effective contact-mode humidity switch 59 with a humidity threshold(RH₀) over a temperature range of T_(j) (a temperature below a nominalor room temperature) to T_(i) (a temperature above a nominal or roomtemperature), as shown in FIG. 10A. Each parallel switching element 58in FIG. 10A includes a bimorphic contact-mode humidity sensing element(e.g., of the type shown in FIGS. 7 and 8) and a bimorphic contact-modetemperature sensing element (e.g., of the type shown in FIGS. 3 through5). The humidity and temperature sensing elements are designed and puttogether in series or shunt configuration such that they provide aneffective contact at a humidity level of RH₀ and temperature of T_(n),where T_(j)<T_(j)<T_(i). The lengths of the bimorphic beams (e.g., 42)of the humidity sensing elements are sized to compensate for the changesin humidity level at which they contact their open contacts (e.g., 48)due to temperature. For example, FIG. 10B shows a case where the CTE ofthe upper film 44 of a beam 42 is lower than the CTE of the lower film46 of the beam 42, resulting in decreased sensitivity as the temperatureincreases if beam length is maintained constant. The humidity switchlevel at higher temperatures can be maintained constant if the beamlength is increased. Again, in FIG. 10B, the beam length of a humiditysensing element 28 is increased to maintain the switch level of RH₀ at ahigher temperature T_(n). The temperature sensing element 28 is added inseries to ensure that the contact of the humidity switch 28 is onlydetected by the system integrated circuitry 16 when the temperatureexceeds T_(n), so that a false humidity switch contact at lower humiditylevels (lower than RH₀) occurring at lower temperatures (lower thanT_(n)) will not be observed by the circuitry 16. The temperature sensingelement 26 in FIG. 10B is a “hot” contact where a contact is made whentemperature goes above the nominal temperature (e.g., as shown in FIG.4). Also, it should be noted, that the described temperaturecompensation is designed such that it is consistent with the preferredoperation of the system interface electronics 16 in which, as discussedlater, the digital output of a sensing element resulting from contactcaused by the highest sensed environmental condition (e.g. humidity) isprocessed in lieu of the remaining elements at any given instant oftime.

FIG. 10C represents a temperature-compensated humidity sensing element28 with the same bimorphic film arrangement as FIG. 10B, but fortemperatures below a nominal (room) temperature. In this case, the beamlength of the humidity sensing element 28 is reduced to maintain theswitching level at RH₀ at lower temperatures. In this case, there is noneed for a temperature sensing element in series with the humiditysensing element 28. However, a shunt temperature sensing element 26configured to switch at relatively cold temperatures (e.g., temperatureslower than nominal, such as the case shown in FIG. 5) is added. Whenclosed, the temperature sensing element 26 nulls the effect of a closedhumidity sensing element 28. This is obtained by making electricalconnections at nodes A, B, C to appropriate voltages and elements, asrepresented in FIG. 10C with reference to a preferred front-end circuit60 shown in FIG. 13. The role of the shunting temperature sensingelement 26 is to ensure that if the temperature is reduced below T_(n),the effective contact between points A and B (which is defined as acontact detected by the front-end circuit 60) remains open and, similarto the case shown in FIG. 10B, an incorrect detection of the humiditysensing element 28 switching at combined lower humidity levels and lowertemperature levels is avoided.

FIGS. 10D and 10E show similar temperature compensation for the case inwhich the CTE of the upper film 44 is greater than that of the lowerfilm 46 of the beam 42, and therefore the inverse of the case shown inFIGS. 10B and 10C. A shunt temperature sensing element 26 is used whenthe temperature increases (FIG. 10D) and a series temperature sensingelement 26 is used when the temperature decreases (FIG. 10E).

FIG. 11 is a simplified block diagram of suitable system integratedcircuitry for the interface electronics 16 of FIG. 1. In addition toproviding the desired cumulative heat and moisture sensing capability byinterfacing with the sensor arrays 14, the interface electronics 16 alsopreferably controls the power dissipation and operation life of the tag10. In order to fully benefit from the high-density array of sensingelements 26 and 28 of the sensor arrays 14, the interface electronics 16is preferably ultra-low power, compact and uncomplicated, employs thefront-end circuitry 60 to detect the mechanical switching of theelements 26 and 28, such as resistively or capacitively, and thenemploys a divider and logic circuit 61 to convert such switching to anelectrical cumulative digital signal. The interface electronics 16 isshown as having counters 52 and 54 for heat and humidity measurements,respectively. The output of the counters 52 and 54 is represented asbeing transmitted with a RFID transceiver 56 and the antenna 22, whichform a passive RFID link to an external reader/interrogator (not shown).Alternatively, an active RFID link could be employed without asignificant impact on the life of the tag 10 provided that a low bitrate and small duty-cycle transmission is performed.

In FIG. 11, power saving is accomplished with the interface electronics16 by using an efficient counter-based implementation for cumulativesense exposure parameter measurements without any need for a complexArithmetic Logic Unit (ALU) or microprocessor. In the preferredembodiment, the temperature and humidity counters 52 and 54 calculatecumulative measurements as the weighted integral of the sensedenvironmental condition over time. In a discrete digital implementation,this is equivalent to output of a real-time clock counter multiplied bythe sensed parameter amplitude. The product of this multiplicationoperation determines the slope of the final output of the counters 52and 54. Provided that a higher clock speed signal is available to theelectronics 16 from the clock generator 18, which is brought down to thereal-time clock by the divider and logic circuit 61, the same functioncan be implemented by controlling the dividing factor without any needfor a multiplier. Such an operation is shown in FIG. 12, where theoutput of one of the counters 52 or 54 with controlled clock speed isshown as directly providing the cumulative heat or humidity output. In apreferred implementation of this approach, the clock speed correspondingto each sensing element 26/28 is higher for elements 26/28 having higherthresholds to the environmental condition they sense. Using thetemperature sensing elements 26 as an example, the clock speed isincrementally faster to each element 26 in relation to the thresholdtemperature of that element 26. This relationship is preferably notproportional, but instead nonlinear, though it is foreseeable that aproportional relationship could be used. The advantage is that thecumulative system output resulting from adding (integrating) theindividual digital outputs of all operational sensing elements 26 willemulate the relative damage that can occur as a result of increasinglyexcessive environmental conditions such as temperature and humidity.While this approach is uncomplicated, it provides a controlimplementation that is both area and power efficient. The output of thesensor arrays 14 is processed digitally and provides the control for thedividing factor. The clock generator 18 is preferably crystal-based anddissipates a few micro amps continuously. An example of a suitable clockgenerator 18 is existing single-chip low-power clock generators used inwristwatches. Alternatively, multiple clock generators could be used tominimize the number (or avoid the use) of real-time clockdivider/counters needed for the cumulative measurements desired by thisinvention.

An important feature for achieving the desired ultra-low powerdissipation and array circuitry compactness is the sensor arrayfront-end circuitry 60. FIG. 13 represents a design for this circuitry60 that employs only five transistors. M_(b) is biased in thesub-threshold regime to set the current I_(b) at a suitable level, suchas not more than 1000 pA. Such a low current guarantees low-powerdissipation and also reduces the risk of damage to the sensing elements26 and 28 attributable to micro-fusing, which could be potentiallycaused by large currents. The mechanical switching of the elements 26and 28 pulls the output inverter input low and results in a high signal.M_(s) is added to cut the current path of the lower range elements 26and 28 when higher temperatures or humidity levels are present and thecorresponding sensing elements 26 and 28 are activated.

The arrays of temperature and humidity sensing elements 26 and 28 of thesensor arrays 14 are preferably monolithically integrated with theCMOS-based interface electronics 16 by fabricating the sensing elements26 and 28 and the electronics 16 on the same CMOS wafer substrate 36, asevident from FIG. 2. Thus, in the preferred embodiment all fabricationsteps for the sensing elements 26 and 28 are preferably compatible withpost-CMOS processing. In addition, the fabrication process is preferablyhigh-yield and compatible with standard semiconductor and MEMSmanufacturing tools. FIGS. 14 through 20 represent a suitablefabrication process, which is shown in FIG. 14 as starting with a dryetch (e.g., reactive ion etch) of a dielectric layer 62 on the substrate36 to expose top metal. This step is overetched slightly (for example,less than 1000 Angstroms) to form contact & anchor metal stands 64. Asacrificial layer 66, such as a photoresist or organic material, is thenspun, baked, and patterned to define anchor regions 68 for the sensingelements 26 or 28, as shown in FIG. 15. The lower film 34 of thetemperature sensing elements 26 (for example, aluminum or gold) is thendeposited and patterned, followed by deposition of a layer 70 (FIG. 16)that when patterned will form the upper film 32 of the temperaturesensing elements 26 and the lower film 46 of the humidity sensingelements 28. This common layer 70 (for example, aluminum or gold,depending on the material of the lower film 34) can be deposited bysputtering, and seals and protects the sacrificial layer 66 fromsubsequent polymer etches. FIG. 17 represents the result of depositing,baking, and patterning the upper film 44 (for example, polyimide) of thehumidity sensing elements 28, followed by the deposition and patteringof a thin protective layer 72 on the upper film 44, such as sputteredsilicon, dielectric deposited by plasma-enhanced chemical vapordeposition (PECVD), or a metal layer. The common layer 70 is thenpatterned and etched to define the upper films 32 of the temperaturesensing elements 26 and the lower films 46 of the humidity sensingelements 28. FIG. 18 shows the completion of the preceding steps, aswell as the forming of a bonding ring 74 that will be employed in thefinal package encapsulation process. In FIG. 19, a photo-resist has beendeposited and patterned to form a sacrificial layer 76 on which metalhas been deposited and patterned to form the top contact pairs 38 forthe temperature sensing elements 26. Finally, the beams 30 and 42 arereleased by etching the sacrificial layers 66 and 76, along with removalof the protective layer 72 by dry etching.

In should be again noted at this point that, in addition to thematerials noted above, the bimorph beams 30 and 42 can be fabricatedfrom various metal and nonmetal materials deposited at suitabletemperatures (preferably less than 400° C.). Suitable materials includethose used in semiconductor and MEMS processing, and are therefore knownto those skilled in the art. Examples include various dielectric layers,different forms of silicon, and other deposited semiconductor layers. Inaddition, suitable sacrificial layers used in the process describedabove can be formed with thin-film low temperature deposited metals,dielectric layers, different forms of silicon, and/or other depositedsemiconductor materials. Accordingly, those skilled in the art candetermine various suitable combinations of sacrificial and structurallayers such that a bimorph beam can be fabricated to have a desiredbimorph response to temperature, humidity, or another environmentalcondition of interest. It should be appreciated that, as with otherknown MEMS processes, a key factor in determining the compatibility ofsacrificial and structural layers during fabrication is the ability toremove the sacrificial layers without attacking or damaging the bimorphstructural layers. The selection of sacrificial layers that can beremoved by a dry etch process is also desirable to minimize stiction ofthe beams 30 and 42 during fabrication.

Packaging of the sensor tag 10 is necessary to permit the use of the tag10 in applications where a robust sensor is desired. Suitable packagingapproaches are preferably compatible with the standard semiconductor andelectronic packaging procedures and equipment. In FIGS. 21 through 24, apackaging scheme is represented that provides for protective packagingand encapsulation of the arrays of sensing elements 26 and 28, alongwith the interface electronics 16, at wafer level to form thesensor-circuit package 50 of FIG. 2. This scheme is a low-cost batchprocess after which the encapsulated integrated sensor array package 50can be integrated into the tag 10 similar to an IC chip. The finalpackaging scheme also encompasses packaging of the sensor-circuitpackage 50 with the clock generator 18 (or another suitable timingcomponent), antenna 22, and battery 24. While wafer level capping andpackaging are often performed using glass frit bonding, packaging costscan be further reduced by using an alternative approach that minimizesthe size of the sealing ring (which is several hundred micrometers inwidth in the case of glass frit bonding). Because the leads to theexterior of the sensor-circuit package 50 can be provided by CMOS metallayers that are buried under a planarized chemically mechanicallypolished (CMP) CMOS dielectric layer, the packaging lead transferrequirements for the package 50 are relaxed to a great extent.

FIGS. 21 through 24 depict the primary fabrication processing steps ofthe sensor-circuit package 50. In FIG. 21, environment access holes 82are patterned and etched from the frontside of a capping wafer 80 toeventually provide access to the environment as required by the humiditysensing elements 28. FIG. 22 shows the result of forming a solder area84 by depositing and patterning an optional solder mold and thin filmsolder on the backside of the capping wafer 80 for the purpose ofbonding the wafer 80 to the sensor-circuit substrate 36. The solder area84 is then protected (not shown) and a recess 86 is etched in thebackside of the capping wafer 80. The recess 86 can be formed by eithera wet or dry etch process. If a deep dry silicon etch is performed, thewalls of the recess 86 will be nearly vertical, as opposed to thesloping walls shown in FIG. 23. Finally, the capping wafer 80 is placedand aligned on the sensor-circuit substrate 36, and solder bonding isperformed. It should be understood that alternative bonding techniquesbesides solder bonding could be used, including glass frit, eutecticbonding, thermo-compression bonding, Transient Liquid Phase (TLP)bonding, or other alloy formation bonds.

From the foregoing, it can be appreciated that the present inventionprovides a sensor tag 10 whose advantages include an array of ultra-lowpower MEMS humidity and temperature sensing elements 26 and 28 capableof very low total power dissipation and direct digital output. Eacharray of sensing elements 26 and 28 directly converts either thermalenergy or environment moisture sorption to a mechanical motion using abimorph structure, without the need for any power other than a low-powerdigital front-end circuitry 60 that dissipates the power of the sensingelements 26 and 28 and converts their switching outputs to a digitalelectrical output. This direct digital output, which provides acumulative measurement capability using counters, precludes any need forpower demanding analog-to-digital conversion circuitry or an ALU. Ifdesired or necessary, humidity sensor temperature compensation can alsobe achieved with an optional ultra low-power scheme that uses the seriescombination of humidity and temperature sensing elements 26 and 28.

As also evident from the above, a batch process can be used to providedirect post-CMOS fabrication, integration and wafer-level packaging ofarrays of the sensing elements 26 and 28 with the system interfaceelectronics 16 to further minimize cost and size, and also enable theformation of cost-effective large arrays of sensing elements 26 and 28without any concern for the need of a large number of interconnects tothe interface electronics 16. Suitable processing steps are fullycompatible with standard semiconductor manufacturing tools and CMOSbackend processing.

Finally, the present invention provides for considerable yieldenhancement for low-cost sensor fabrication through the use of verylarge arrays of the compact sensing elements 26 and 28 and front-endcircuitry 60, and selecting a subset of each array of elements 26 and 28on the basis of desired operating parameters ascertainable by acost-effective, high-throughput post-manufacturing test. As such, thepresent invention avoids conventional sensor manufacturing where thetolerance of the manufacturing processes are tightened and postfabrication calibration are employed to perfect the operating parametersof individual sensors. This approach is enabled by the low-costminiature fully-integrated sensor structure of this invention that makesthe inclusion of redundant sensing elements in a sensor array costeffective.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

1. A micro-electro-mechanical digital sensing system comprising: aplurality of sensing elements on a substrate, each sensing elementcomprising a cantilevered bimorph beam and at least one set of opencontacts configured for non-latching contact-mode operation with thebimorph beam, the bimorph beam of each sensing element being responsiveto changes in an environmental condition so as to deflect toward andaway from the open contacts thereof in response to the changes in theenvironmental condition, the bimorph beams and the sets of open contactsbeing configured to enable the bimorph beams to contact and close theirrespective sets of open contacts in response to a first change in theenvironmental condition without latching their respective sets of opencontacts, the bimorph beams being configured to contact and close theirrespective open contacts at different levels of the environmentalcondition, each of the sensing elements producing a digital output whenthe bimorph beam thereof contacts and closes the open contacts thereof;and an integrated circuitry interfacing with the sensing elements sothat the digital outputs of the sensing elements are processed togenerate a system output of the sensing system, wherein the integratedcircuitry is configured to generate the system output as a time-weightedcumulative output generated on the basis of the digital outputs of thesensing elements over time.
 2. The micro-electro-mechanical digitalsensing system according to claim 1, wherein the environmental conditionto which the bimorph beams are responsive is at least one environmentalcondition chosen from the group consisting of temperature, relativehumidity, chemicals, shock/vibration, tilt, pressure, acceleration, andbiological agents.
 3. The micro-electro-mechanical digital sensingsystem according to claim 1, further comprising means for resistively orcapacitively sensing contact between the bimorph beams and theirrespective open contacts.
 4. The micro-electro-mechanical digitalsensing system according to claim 1, wherein the integrated circuitrycomprises front-end circuitry that directly interfaces with the sensingelements and detects contacts between the bimorph beams and theirrespective open contacts.
 5. The micro-electro-mechanical digitalsensing system according to claim 1, wherein the integrated circuitry isconfigured to generate the system output by individually processing thedigital outputs of the sensing elements that at a given time contact andclose their respective open contacts in response to a maximum level ofthe environmental condition at the given time.
 6. Themicro-electro-mechanical digital sensing system according to claim 1,wherein the bimorph beams of a first set of the sensing elements areresponsive to temperature and the bimorph beams of a second set of thesensing elements are responsive to relative humidity.
 7. Themicro-electro-mechanical digital sensing system according to claim 1,further comprising a battery for powering the integrated circuitry. 8.The micro-electro-mechanical digital sensing system according to claim1, further comprising a radio frequency identification link operable towirelessly transmit the system output.
 9. The micro-electro-mechanicaldigital sensing system according to claim 1, wherein the bimorph beamsof the sensing elements are responsive to the changes in theenvironmental condition without any power supplied to the bimorph beams.10. The micro-electro-mechanical digital sensing system according toclaim 1, wherein the integrated circuitry generates the system output asa digital system output directly from the digital outputs of the sensingelements without performing any analog-to-digital conversion.
 11. Themicro-electro-mechanical digital sensing system according to claim 1,further comprising at least one clock that generates at least one clockspeed, and the integrated circuitry uses the clock speed to generate thetime-weighted cumulative output.
 12. The micro-electro-mechanicaldigital sensing system according to claim 11, wherein the integratedcircuitry is configured to generate the time-weighted cumulative outputby individually processing the digital outputs of the sensing elementsthat at a given time contact and close their respective open contacts inresponse to a maximum level of the environmental condition at the giventime, and is further configured to integrate the digital outputs overtime using the clock speed of the clock without use of an arithmeticlogic unit.
 13. The micro-electro-mechanical digital sensing systemaccording to claim 1, further comprising at least one clock thatgenerates multiple different clock speeds that are assigned to thedigital outputs of the sensing elements to generate the system output.14. The micro-electro-mechanical digital sensing system according toclaim 13, wherein the integrated circuitry is configured to associatehigher clock speeds with the digital outputs of the sensing elementsresponsive to higher levels of the environmental condition.
 15. Amicro-electro-mechanical digital sensing system comprising: a pluralityof sensing elements on a substrate, each sensing element comprising acantilevered bimorph beam and at least one set of open contactsconfigured for non-latching contact-mode operation with the bimorphbeam, the bimorph beam of each sensing element being responsive tochanges in an environmental condition so as to deflect toward and awayfrom the open contacts thereof in response to the changes in theenvironmental condition, the bimorph beams and the sets of open contactsbeing configured to enable the bimorph beams to contact and close theirrespective sets of open contacts in response to a first change in theenvironmental condition without latching their respective sets of opencontacts, the bimorph beams being configured to contact and close theirrespective open contacts at different levels of the environmentalcondition, each of the sensing elements producing a digital output whenthe bimorph beam thereof contacts and closes the open contacts thereof;and an integrated circuitry interfacing with the sensing elements sothat the digital outputs of the sensing elements are processed togenerate a system output of the sensing system; wherein the bimorphbeams of a first set of the sensing elements are responsive totemperature and the bimorph beams of a second set of the sensingelements are responsive to relative humidity, and the sensing systemfurther comprises means for temperature compensation of the second setof sensing elements.
 16. The micro-electro-mechanical digital sensingsystem according to claim 15, wherein the temperature compensation meanscomprises temperature switches electrically connected to the second setof sensing elements.
 17. The micro-electro-mechanical digital sensingsystem according to claim 16, wherein each of the temperature switchescomprises a cantilevered bimorph beam between at least two sets of opencontacts, the bimorph beams of the temperature switches being responsiveto temperature.
 18. The micro-electro-mechanical digital sensing systemaccording to claim 16, wherein the temperature switches are electricallyconnected to the second set of sensing elements in series and in shuntconfigurations.
 19. The micro-electro-mechanical digital sensing systemaccording to claim 16, wherein the second set of sensing elementscomprises subsets of the sensing elements connected in parallel, eachsubset defines a humidity switch, and each of the sensing elementswithin each humidity switch is electrically connected to a correspondingone of the temperature switches.
 20. The micro-electro-mechanicaldigital sensing system according to claim 19, wherein the sensingelements within each humidity switch are configured to contact and closetheir respective open contacts at different levels of relative humidity.21. The micro-electro-mechanical digital sensing system according toclaim 20, wherein the sensing elements and the temperature switcheswithin each humidity switch cooperate to yield a single digital signalover a limited temperature range determined by the temperature switches.22. The micro-electro-mechanical digital sensing system according toclaim 21, wherein the temperature switches are electrically connected totheir respective sensing elements in series and in shunt configurations.23. A method of sensing an environmental condition, the methodcomprising the steps of: providing a sensing system comprisingintegrated circuitry and a plurality of non-latchable contact-modesensing elements, the sensing elements being responsive to theenvironmental condition and being operable to close a plurality of pairsof open contacts at different levels of the environmental condition toindividually produce digital outputs, the sensing elements and the pairsof open contacts being configured to enable closing without latching thepairs of open contacts; and interfacing the sensing elements with theintegrated circuitry so that the digital outputs of the sensing elementsare processed to generate a system output of the sensing system, whereinthe integrated circuitry generates the system output as a time-weightedcumulative output generated on the basis of the digital outputs of thesensing elements over time.
 24. The method according to claim 23,wherein the environmental condition is at least one environmentalcondition chosen from the group consisting of temperature, relativehumidity, chemicals, shock/vibration, tilt, pressure, acceleration, andbiological agents.
 25. The method according to claim 23, wherein theintegrated circuitry generates the system output by individuallyprocessing the digital outputs of the sensing elements that at a giventime contact and close their respective open contacts in response to amaximum level of the environmental condition at the given time.
 26. Themethod according to claim 23, wherein at least some of the sensingelements are responsive to temperature and at least some of the sensingelements are responsive to relative humidity.
 27. The method accordingto claim 23, wherein a first set of the sensing elements are responsiveto temperature and a second set of the sensing elements are responsiveto relative humidity.
 28. The method according to claim 23, furthercomprising wirelessly transmitting the system output via a radiofrequency identification link.
 29. The method according to claim 23,wherein the integrated circuitry generates the system output as adigital system output directly from the digital outputs of the sensingelements without performing any analog-to-digital conversion of thedigital outputs of the sensing elements.
 30. The method according toclaim 23, further comprising the steps of: testing the sensing elementsto correlate the digital outputs thereof to the environmental condition;identifying a subset of the sensing elements that produce digitaloutputs corresponding to a predetermined range of levels of theenvironmental condition, a remainder of the sensing elements producingdigital outputs corresponding to a wider range of levels of theenvironmental condition than the predetermined range; and then causingthe integrated circuitry to interface with only the subset of thesensing elements so that the system output is generated using thedigital outputs of only the subset of the sensing elements while thedigital outputs of the remainder of the sensing elements are ignored bythe integrated circuitry when generating the system output.
 31. Themethod according to claim 23, wherein the sensing system comprises atleast one clock that generates at least one clock speed, and theintegrated circuitry uses the clock speed to generate the time-weightedcumulative output.
 32. The method according to claim 31, wherein theintegrated circuitry generates the time-weighted cumulative output byindividually processing the digital outputs of the sensing elements thatat a given time contact and close their respective open contacts inresponse to a maximum level of the environmental condition at the giventime, and is further configured to integrate the digital outputs overtime using the clock speed of the clock without use of an arithmeticlogic unit.
 33. The method according to claim 23, wherein at least oneclock generates multiple different clock speeds that are assigned to thedigital outputs of the sensing elements to generate the system output.34. The method according to claim 33, further comprising assigninghigher clock speeds to the digital outputs of the sensing elementsresponsive to higher levels of the environmental condition.
 35. A methodof sensing an environmental condition, the method comprising the stepsof: providing a sensing system comprising integrated circuitry and aplurality of non-latchable contact-mode sensing elements, the sensingelements being responsive to the environmental condition and beingoperable to close a plurality of pairs of open contacts at differentlevels of the environmental condition to individually produce digitaloutputs, the sensing elements and the pairs of open contacts beingconfigured to enable closing without latching the pairs of opencontacts; and interfacing the sensing elements with the integratedcircuitry so that the digital outputs of the sensing elements areprocessed to generate a system output of the sensing system; wherein afirst set of the sensing elements is responsive to temperature and asecond set of the sensing elements is responsive to relative humidity,and the method further comprises temperature compensating each of thesecond set of the sensing elements with a temperature switchelectrically connected therewith.
 36. The method according to claim 35,wherein the temperature switches are electrically connected to thesecond set of sensing elements in series and in shunt configurations.37. The method according to claim 35, wherein the second set of sensingelements comprises subsets of the sensing elements connected inparallel, each subset defines a humidity switch, and each of the sensingelements within each humidity switch is electrically connected to acorresponding one of the temperature switches.
 38. The method accordingto claim 37, wherein the sensing elements within each humidity switchcontact and close their respective open contacts at different levels ofrelative humidity.
 39. The method according to claim 38, wherein thesensing elements and the temperature switches within each humidityswitch cooperate to yield a single digital signal over a limitedtemperature range determined by the temperature switches.
 40. The methodaccording to claim 39, wherein the temperature switches are electricallyconnected to their respective sensing elements in series and in shuntconfigurations.
 41. A method of producing a micro-electro-mechanicaldigital sensing system, the method comprising the steps of: fabricatinga sensing system comprising integrated circuitry and a plurality ofcontact-mode sensing elements, the sensing elements being responsive toan environmental condition and operable to close at least one pair ofopen contacts at different levels of the environmental condition toindividually produce digital outputs; determining responses of thesensing elements to different levels of the environmental condition;selecting a subset of the sensing elements that produce digital outputscorresponding to a predetermined range of levels of the environmentalcondition, a remainder of the sensing elements producing digital outputscorresponding to a wider range of levels of the environmental conditionthan the predetermined range; and then configuring the integratedcircuitry to monitor and process the digital outputs of only the subsetof the sensing elements with the integrated circuitry to generate asystem output of the sensing system while the digital outputs of theremainder of the sensing elements are ignored by the integratedcircuitry when generating the system outputs; wherein the sensing systemis produced without physically modifying and without electronicallycompensating any of the sensing elements to alter the responses of thesensing elements to the environmental condition after the selectingstep.
 42. The method according to claim 41, wherein the environmentalcondition to which the sensing elements are responsive is at least oneenvironmental condition chosen from the group consisting of temperature,relative humidity, chemicals, shock/vibration, tilt, pressure,acceleration, and biological agents.
 43. The method according to claim41, wherein at least some of the sensing elements are fabricated to beresponsive to temperature and at least some of the sensing elements arefabricated to be responsive to relative humidity.
 44. The methodaccording to claim 41, wherein the integrated circuitry generates thesystem output as a time-weighted cumulative output generated on thebasis of the digital outputs of the subset of the sensing elements overtime.
 45. The method according to claim 41, wherein the remainder of thesensing elements are responsive to levels of the environmental conditionoutside a range defined by the levels of the environmental condition towhich the subset of sensing elements are responsive.
 46. The methodaccording to claim 41, wherein the sensing system is produced withoutindividually calibrating the sensing elements.